One significant indication of the spatial resolution of an antenna is the antenna beamwidth. The half power beamwidth is defined as the angular separation between the half power points on an antenna radiation pattern, where the gain is one half the maximum value. For a reflector antenna, the half power beamwidth, α, can be expressed as:
                    α        =                              k            ⁢                                                  ⁢            λ                    D                                    Eq        .                                  ⁢        1            
where λ is the wavelength of the received signal, k is a factor that depends on the shape of the reflector and the method of illumination, and D is the diameter of the antenna.
As seen in Equation 1, the half power beamwidth decreases with decreasing wavelength and/or increasing diameter. Accordingly, for an antenna designed for a given wavelength, the limiting factor on antenna performance is the antenna size. It will be appreciated, however, that there are practical limitations to the maximum size of an antenna. This is especially true in mobile communications applications, where the available space and power for an antenna is significantly limited.